Analysis of subsurface formations has led to more efficient oil and gas recovery from hydrocarbon reservoirs. In recent years, exploration and development of hydrocarbon reserves has been occurring at increasingly deeper depths of water. As the water depths increase and the wells that are drilled lengthen, recovery of formation fluids from subsurface formations becomes increasingly difficult and complex. In consequence, vertical seismic profile surveys are a useful and necessary technique to characterize subsurface formations so that the hydrocarbon fluids in the formations can be efficiently recovered.
A vertical seismic profile (VSP) is a class of borehole seismic measurements used for correlation between surface seismic receivers and wireline logging data. VSPs can be used to tie surface seismic data to well data, providing a useful tie to measured depths. Typically, VSPs yield higher resolution data than surface seismic profiles provide. VSPs enable converting seismic data to zero-phase data, as well as enable distinguishing primary reflections from multiples. In addition, a VSP is often used for analysis of portions of a formation ahead of the drill bit.
Narrowly defined, VSP refers to measurements made in a vertical wellbore using acoustic receivers inside the wellbore and a seismic source at the surface near the well. In a more general context as used herein, however, VSPs vary in well configuration, the number and location of sources and acoustic receivers, and how they are deployed. Nevertheless, VSP does connote the deployment of at least some receivers in the wellbore. Most conventional VSPs use a surface seismic source, which is commonly a vibrator on land, or an air-gun in marine environments. There are various VSP configurations including zero-offset VSP, offset VSP, walkaway VSP, vertical incidence VSP, salt-proximity VSP, multi-offset VSP, and drill-noise or seismic-while-drilling VSP.
Check-shot surveys are similar to VSP in that acoustic receivers are placed in the borehole and a surface source is used to generate an acoustic signal. However, a VSP is more detailed than a check-shot survey. The VSP receivers are typically more closely spaced than those in a check-shot survey; check-shot surveys may include measurement intervals hundreds of meters apart. Further, a VSP uses the reflected energy contained in the recorded trace at each receiver position as well as the first direct path from source to receiver, while the check-shot survey uses only the direct path travel time.
Although VSPs are a valuable information tool for analyzing subsurface formations, acquisition of seismic survey data by way of seismic waves that are generated by a surface source poses problems. In particular, use of a surface source, such as an air-gun, in marine environments generates seismic energy waves that must propagate a distance through water to reach the seabed and the subsurface formations below the seabed for purposes of seismic survey data. In this, the distance between a surface seismic energy source and the seabed introduces uncertainty in the acquired seismic data. Furthermore, surface seismic sources for generating seismic waves in marine environments, such as air-guns, are affected by sea swells in rough weather. As an additional shortcoming, conventional surface and subsea seismic sources require a flow of high-pressure air and/or hydraulic oil for their operation. Typically, such conventional seismic sources require special procedures for their safe handling and operation.
Among devices that are known for generating seismic waves below the sea surface are powered seabed sources, such as shear wave air-guns. Such devices have been used in borehole seismic surveys. However, powered seabed sources require the delivery of large amounts of energy from the sea surface down to the seabed floor, which makes the operation and handling of such devices complicated and expensive. In this, it is difficult to safely deploy and retrieve a powered source from the seabed after acquiring seismic data.
Boro, C. et al., Sphere Implosion Initiation Device, Record of Invention IL-10551, University of California, LLNL Patent Group (1999), use imploding spheres in calibration of hydro-acoustic monitoring devices used for the Comprehensive Nuclear Test Ban Treaty. Orr, M. et al., Acoustic Signatures From Deep Water Implosions of Spherical Cavities, J. Acoust. Soc., V 59, pp. 1155-1159 (1976), and Pulli J. J. et al., Hydroacoustic Calibration with Imploding Glass Spheres, 22nd Annual DoD/DOE Seismic Research Proc., Vol. III, pp. 65-74 (2000), also discuss imploding spheres.